Natural period of reinforced concrete building frames on pile foundation considering seismic soil-structure interaction effects

Abstract

The magnitude of seismic forces induced within a building, during an earthquake, depends on its natural period of vibration. The traditional approach is to assume the base of the building to be fixed for the estimation of the natural period and ignore the influence of soil-structure interaction (SSI) citing it to be beneficial. However, for buildings resting on soft soils, the soil-foundation system imparts flexibility at the base of the building and it is imperative for SSI effects to exist that may prove to be detrimental. Presence of SSI effects modifies the seismic forces induced within the building, which is dependent on the change in its fixed base natural period. Expressions for determination of the natural period of a structural system under the influence of SSI (effective natural period) are available in the literature. Although useful and made applicable to all types of foundation systems, these expressions are developed either using simplified models or are applicable to shallow foundations and may not be suitable to all types of structural-foundation systems in general. This article investigates the influence of seismic soil-structure interaction on the natural period of RC building frame supported on pile foundations. Detailed finite element modelling of an exhaustive number of models, encompassing various parameters of the structure, soil, and the pile foundation has been carried out in OpenSEES to study the effect of SSI on the natural period of the RC building frame. The effect of various parameters on the natural period of the building frame under SSI is investigated and the results have been used to develop an ANN architecture model for the estimation of the effective natural period of RC building frame supported by pile foundation. Garson’s algorithm is used to conduct sensitivity analysis for examining the importance of various parameters that govern the determination of effective natural period. A predictive relationship for obtaining the effective natural period has been proposed using ANN architecture in the form of a modification factor that is to be applied on the fixed base natural period, and which depends on various input parameters of the building frame-pile-soil system. A comparison of the proposed relationship with those available in literature demonstrates its usefulness and applicability to RC building frames on pile foundations.